On Wednesday 08 January 2014, Geert Uytterhoeven wrote:
VGA is special, in that it uses "ISA memory space". This is not a subset of "PCI memory space", but something different. Some PCI host bridges (IIRC, e.g. on Mac) do not allow access to this space. Most other "PC I/O" use ISA I/O space, which is a subset of PCI I/O space.
Right, but they often go together, and I think vgacon actually requires both, doesn't it? I'm not aware of anything else requiring access to the 0xa0000-0xfffff or the 0xf00000-0xffffff ISA memory windows except VGA, but I could be missing some less common devices. These are often not available on non-x86 systems, which prevents VGA from working even if low I/O space addresses are routed to PCI.
The CONFIG_PC_IO symbol would mostly be used for stuff like PC-style floppy, dma, rtc, pic, parport, uart, etc. I think it does make sense to include VGA in that list, but we may want to add a few machines that explicitly support VGA on PCI without supporting other PC-style components.
Arnd